The present invention relates generally to the optical elements having at least two optical material layers having different refractive indexes and more specifically to optical elements having at least two layers of the same optical material but with different refractive indexes and a method for fabricating optical elements by inducing changes in the index of refraction in optical materials utilizing large area electron beam radiation.
Various optical elements have a multiple layer structure of layers of different optical materials having different indexes of refraction or a multiple layer coating of layers of different optical materials having different indexes of refraction. These multiple layer optical elements have a variety of uses in total internal reflection, wavelength filters and diffraction.
An optical waveguide carries a light beam along a designated path within the waveguide. The optical waveguide is typically formed by utilizing materials of different refractive index. The inner waveguide is formed of a first optical material having a high index of refraction. The outer cladding layer around the inner waveguide is formed of a second different optical material having a second low index of refraction.
The inner waveguide material typically exhibits high optical transmission for a light beam to maximize the internal reflection of a light beam traveling along the inner layer of the waveguide and to minimize the signal loss of the light beam. The current state of the art of producing these waveguides and producing these materials of different index of refraction is to utilize two different materials, which are layered in an additive or subtractive process.
Similarly, the inner cylindrical core layer of an optical fiber will have a high index of refraction while the surrounding cylindrical cladding layer will have a low index of refraction to maximize the internal reflection of a light beam traveling along the inner core of the optical fiber.
An interference filter is formed by a first layer of high refractive index material on a substrate with a second layer of a low refractive index material on the first layer. The interference filter can be an antireflection coating to reduce reflected light by decreasing the refractive index difference between the substrate next to the first layer and the ambient atmosphere next to the second layer. The interference filter can be a heat reflective thermal control film, which transmits visible radiation while reflecting infrared radiation. The interference filter can also be used to reflect or transmit selected wavelengths of light or reflect or transmit ranges of wavelengths of light.
Alternating layers of high and low refractive index materials can be used as diffraction gratings or beamsplitters.
The index of refraction can vary within a layer or across multiple layers to form gradient index optical elements. Optical waveguides and optical fibers can have gradient indexes. A gradient index lens functions by diffraction from the layers of different refractive indexes, rather than the traditional refraction from the curved surface of a lens made from a single material having a single index of refraction.
The two different materials with different indexes of refraction are structurally and/or chemically distinct and are brought together during the assembly process for the optical element.
Typically, these optical elements are fabricated by chemical vapor deposition of the layers of different optical materials. However this limits the possible optical material layers since the layers must be compatible with fabrication by deposition and affinity for bonding with each other. Similarly, the optical materials may require different exposure times, temperatures, pressures and atmospheres which may alter the other optical material.
In waveguides and optical fibers in particular, an optical adhesive may be mandated to bond the layers of structurally and chemically distinct materials together. The adhesive layer will effect waveguiding in waveguides and optical fibers and also effect transmittance and reflectance if used in other optical elements.
The multiple layers of different materials create problems in fabrication as edge breakage and differential polishing rates between the glue and core/cladding materials must be taken into account as well as controlling appropriate glue thickness.
Separate from the fabrication issue is that the dissimilar optical materials may have different coefficients of thermal expansion which will cause the fabricated optical element to function differently or not at all at different temperatures.
It is an object of the present invention to provide different refractive indexes from the same optical material within an optical element.